1. Field of the Invention
The present invention relates to a fuel cell. More precisely, the present invention relates to a gas diffusion layer of a fuel cell, a manufacturing apparatus and a manufacturing method thereof.
2. Description of Related Art
Along with the industrial development, the consumption of the conventional energy resources such as coal, petroleum, and natural gas continuously increases. Since the reservations of these resources are limited, new alternative energy technology should be developed to substitute the conventional method of energy consumption. The fuel cell is an important new alternative energy technology with practical value.
Briefly speaking, the fuel cell basically is a generator apparatus that uses the reverse reaction of the water electrolysis to convert chemical energy into electric energy. Since the fuel cell has the advantages of low operation temperature, quick start, high energy density, low pollution, and a wide range of application, the fuel cell has a high commercial value. It has become a successively developed and promoted technology all over the world. The commonly seen fuel cell includes phosphoric acid fuel cell (PAFC), direct methanol fuel Cell (DMFC), alkaline fuel cell (AFC), molten carbonate fuel cell (MCFC), solid oxide fuel cell (SOFC), and proton exchange membrane fuel cell (PEMFC).
FIG. 1A is a cross-sectional view of an internal structure of a typical proton exchange membrane fuel cell (PEMFC). As shown in FIG. 1A, the PEMFC mainly includes a proton exchange membrane (PEM) 100, a catalyst layer 102, a gas diffusion layer (GDL) 104, and a bipolar plate 106. During the PEMFC operation, the oxidation reaction of H2 is taking placed at the anode 110, and the reduction reaction of O2 is taking placed at the cathode 112. The reactant gas H2 at the anode 110 is decomposed into hydrogen ions (H+) and electrons (e−) in the presence of a catalyst, as shown in Equation (1). The electrons (e−) escape from the anode 110. It flows through the cell external circuit 114 and load 115, then it reaches the cathode 112. Meanwhile, the hydrogen ions (H+) are transferred from the anode 110 to the cathode 112 through the proton exchange membrane (PEM) 100. The hydrogen ions (H+) and the electrons (e−) combine with the oxygen molecules (O2) at the cathode 112 to produce water (H2O), as shown in Equation (2). Therefore, the overall reaction of the entire fuel cell is: H2 reacts with O2 to produce H2O, as shown in Equation (3).At the anode: H2→2H++2e−  (1),At the cathode: ½O2+2H++2e−→H2O  (2),Overall reaction: H2+½O2→H2O  (3).
The hydrogen ions (H+) are produced at the anode 110. Due to the electrical field in the cell, the H+ is migrated from the anode toward the cathode 112 continuously. During the H+ migration, it drags water molecules (H2O) along to the cathode 112 (i.e., osmosis drag of water, the H+ is migrated in the form of a hydrated ion H+(H2O)n). Therefore, during the cell reaction, the H2O molecules will be continuously transferred from the anode 110 to the cathode 112. If the water cannot be supplied at adequate amount, the proton exchange membrane (PEM) 100 will become excessively dry, the H+ conducting capability of the membrane will be reduced, and the power output of the fuel cell will be significantly reduced. However, a great amount of water (H2O) will be produced by the reduction reaction of O2 at the cathode 112. If the extra water cannot be adequate discharged from the cell, the catalyst layer 102 and the gas diffusion layer 104 at the cathode 112 will be flooded and filled with water. The gas diffusion layer filled with water becomes a diffusion barrier of oxygen. It retards the oxygen getting into the catalyst layer and the cell output power is significantly reduced. Therefore, the requirement on the water management condition at the cathode 112 and that at the anode 110 of the fuel cell are significantly different from each other. Controlling and maintaining of the water balance in the cathode 112 and the anode 110 and keeping the gas transferring freely inside the electrodes, are critical for maintaining the performance of the PEMFC at its optimal condition.
The gas diffusion layer 104 is located between the catalyst layer 102 and the gas flow path 108. It is one of the key components of the fuel cell in the determination of the water balance in the fuel cell. A good gas diffusion layer shall maintain the catalyst layer and membrane at adequate moisture for high ionic conductivity and keep itself at dry condition for good gas diffusion pathway.
FIG. 1B is a schematic view of the internal structure of a gas diffusion layer. As shown in FIG. 1B, the gas diffusion layer 104 has a dual-layer structure therein, one layer is a gas diffusion medium (GDM) 104a. It is a macro-porous carbon fiber substrate. The other one is a micro porous layer (MPL) 104b. It is a micro-porous carbon powder substrate. The MPL is coated on the GDM by a particular manufacturing method. This method usually uses an ultrasonic oscillator to mix highly conductivity carbon powders, dispersing agents, a solution, and a hydrophile/hydrophobicizer, so as to produce a liquid micro porous layer slurry. This slurry is then using a coating technique to coat the micro porous layer slurry on the surface of the gas diffusion medium 104a. After a high temperature sintering, the micro porous layer 104b is obtained.
The gas diffusion layer 104 plays many roles in the fuel cell stack, such as (1) providing a pathway for the reactant gas (H2, O2); (2) providing a pathway for the reaction products (water, heats) to leave the catalyst layer; (3) providing a conducting medium for the electrons current; and (4) acting as a structural support for the catalyst layer and the PEM. Therefore, the gas diffusion layer 104 should have all the characteristics of electric conductivity, thermal conductivity, porosity, gas permeability, hydrophilicity/hydrophobicity, and mechanical strength. As described above, during the reaction of the PEMFC, many water molecules (H2O) are required for the H+ to migrate from the anode 110 to the cathode 112. So that the gas diffusion layer of a hydrophilic material is suitable for being used at the anode 110. A great amount of water (H2O) will be produced by the reduction reaction of O2 at the cathode 112, so that the gas diffusion layer of a hydrophobic material is suitable for being used at the cathode 112. If both the transference of the reactant gases (H2, O2) and the discharge of the water (H2O) should be taken into consideration, the gas diffusion layer made of combining different hydrophilic/hydrophobic materials can be used. Based on the above, the hydrophilic/hydrophobic structure of the gas diffusion layer 104 is the critical factor for affecting the water balance in the fuel cell.
The conventional manufacturing method of the gas diffusion layer only adopts a surface coating process to form a micro porous layer on the surface of the gas diffusion medium, without forming different hydrophilic/hydrophobic structures and channel layers in the gas diffusion medium (GDM). Moreover, the micro porous layer (MPL) has a poor adhesion with the gas diffusion medium, which is easily stripped off. Moreover, the gas diffusion layer cannot meet the requirements on both the high gas permeability/high hydrophilicity and the high gas permeability/high electric conductivity. The gas diffusion layer (GDL) usually has a low electric conductivity at the through plane.